Commercially available passivation methods for white-rust protection of hot-dip galvanized steel have been investigated. The passivations were either based on trivalent chromium or chromium free. A chromate based conversion coating was used for reference. The treated panels were tested with regard to white rust protection and paintability. The surface chemistry of the conversion coatings was monitored with scanning Auger electron spectroscopy and X-ray photoelectron spectroscopy. Coating thicknesses were measured using Auger electron sputter depth profiling. The passivations were applied with a thickness recommended by the supplier and thus showed large variation. The thickness of the chromium free passivation (Cr-free) is approximately 75 nm. The coating contains the active ions; H3O+, Ti4+, Mn2+, Zn2+, PO4 3-. The passivation based on trivalent chromium (Cr-III) is approximately 30 nm thick and contains the active ions; H3O+ Cr3+, PO4 3-, F. The chromate based passivation (Cr- VI) is approximately 5 nm thick and contains the active ions Cr6+/Cr3+, F-. The Cr-free and the Cr-III passivations showed similar white rust protection in the corrosion tests. The corrosion resistance was good although it did not fully reach the level of the Cr-VI passivation. The results from the tests of the painted panels showed that the powder paint worked well on all three passivations. The solvent born paint worked best on the passivation based on trivalent chromium. The water born paint showed poor resistance to blistering in the Cleveland humidity test for all three passivations. In this test the passivation with hexavalent chromium showed slightly better results than the chromate free passivations.

The influence of dipping temperature and time on the surface chemistry of hot-dipped galvanized steel sheets during the alkaline degreasing process is investigated. The surface chemistry was monitored with scanning Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The results show high Al concentrations on the untreated surfaces, which are significantly reduced during alkaline degreasing. The same conclusions could be drawn for the carbon compounds that accumulate on the surface during storage. The measurements reveal a gradual reduction in surface Al as the alkali solution temperature and/or degreasing time are increased. When degreasing was conducted at 70 °C for 30 s the surface was practically free from Al, which was present only in small islands. Furthermore, the experiments showed that the thickness of the oxide film covering the surfaces before and after alkaline degreasing is approximately 20 Å. The main constituents of the film varied from ZnAl hydroxide/oxide to Zn hydroxide/oxide, before and after degreasing, respectively.

In this paper the bonding of thin vegetable oil films on mercapto silane treated aluminium surfaces has been studied. The silane molecules are attached to the surface by metal–oxygen–silicon bonds. The coupling between the unsaturated bonds of the vegetable oil and the thiol functionalised surface was obtained through a photoinduced thiol-ene reaction. The surfaces were characterised by X-ray photoelectron spectroscopy (XPS). Vegetable oil contains both saturated and unsaturated carbon chains. For the reactions investigated in this study it is the unsaturated carbon chains that can react by a thiol-ene reaction and the results indicate that it is possible to attach a vegetable oil to a metal surface pre-treated with a thiol functionalised silane.

ToF-SIMS has been used to analyse tribological induced chemical changes of organic coatings deposited on steel strip hot-dip coated with a 55.0% Al–43.4% Zn–1.6% Si alloy (Aluzink). The organic coating was a styrene–acrylic co-polymer containing different forming additives. The forming properties of the organic coatings were evaluated with modified scratch testing. The friction curves show that organic coated hot-dip coated steel displays significantly better tribological properties, i.e. lower coefficient of friction and lower wear, as compared to hot-dip coated steel. Furthermore, the organic coatings showing the highest material transfer tendency also show the highest wear. ToF-SIMS spectra show that a transfer film consisting of species from the organic coating is formed on the ball counter surface. Finally, a combination of SEM and ToF-SIMS analysis shows that mechanical failure of the coating dominates, i.e. no tribochemical changes of the coatings could be detected in the wear track.

9.

Bexell, Ulf

et al.

Dalarna University, School of Technology and Business Studies, Materials Technology.

In the present work an organofunctional silane, gamma-mercaptopropyltrimethoxysilane (gamma-MPS), has been deposited on hot-dip galvanized cold rolled steel from different silane solution concentrations. Painted and unpainted silane treated samples were corrosion tested and painted samples were adhesion tested. The surface chemistry of the unpainted silane treated samples was investigated with AES, ToF-SIMS and EDS and the surface morphology was studied with SEM. The results show that the silane film thickness is dependent on the silane concentration in the silane solution and a higher silane concentration gives a thicker film. Moreover, thicker films tend to give films with a pronounced crack pattern and even detachment of film debris. Corrosion tests of unpainted samples show that gamma-MPS can not work as a passivation treatment but gives a very good adhesion to the paint

Today there exist many ferritic stainless steel grades with a chemical composition speciallydesigned to be used as interconnects in solid oxide fuel cell applications in a temperatureinterval of 650-850°C. The steels have good high temperature mechanical properties andcorrosion resistance as well as good electron conductivity in the formed chromium oxidescale.One way to substantially decrease the high temperature degradation of the interconnectsteel i.e. improve properties such as increased surface conductivity and decreasedoxidation and chromium evaporation is to coat the interconnect steel with suitablecoatings. Today it is well known that a thin cobalt coating hinders chromium evaporationand a ceria coating lowers the oxidation rate at high temperature. Thus, by coating theinterconnect steel the properties are improved to an extent that it should be possible to usea cheaper standard steel, e.g. AISI 441, as substrate for the coatings.In this study the ferritic stainless steel alloys Sandvik Sanergy HT and AISI 441 is oxidizedin laboratory air at temperatures at 750°C, 800°C and 850°C. The results show that a welladhered oxide scale of a complex layered structure is formed with significant amounts ofMn, Fe, Cr and Ti in the oxide scale. A Ce coating significantly reduces the growth rate ofthe oxide scale. The lower Cr content in the AISI 441 alloy does not affect the initial hightemperature corrosion properties when coated with Ce. Also, the results demonstrate theusefulness of ToF-SIMS depth profiling for characterisation of the initial stages of oxidationof SOFC materials.

16.

Bexell, Ulf

et al.

Dalarna University, School of Technology and Business Studies, Material Science.

Olsson, Mikael

Dalarna University, School of Technology and Business Studies, Material Science.

In this study, thin sheets of a 22% Cr ferritic steel, Sandvik Sanergy HT has been coated with a thin metallic cobalt film. Samples of the coated steel sheet were exposed to two different forming experiments simulating different aspects of sheet metal forming and post high temperature oxidation. The results show that the metallic cobalt film cracks when the steel sheet is subjected to biaxial straining but that the cracks heal at high temperature. Material subjected to modified scratch testing i.e. simulating the contact between sheet metal and forming tool will oxidize in the same manner as non-scratched material. Also, from high temperature corrosion point of view there is no difference between materials formed under lubricated or dry conditions when oxidized. However, from a tribological point of view a lubricated contact situation is preferred.

The attempt to decrease the temperature in solid oxide fuel cells has made it possible to use metallic materials as interconnect, i.e. the part that separates the anode and the cathode in a fuel cell stack. Besides being impermeable to gases the interconnect give good electron conduction between adjacent cells and to the external circuit. Thus, the unavoidable oxide scale must have good electron conductivity and therefore good adhesion to the metal since spalling of the oxide scale from the metal will give rise to higher resistance in the whole oxide scale metal system.
Scratch testing is today a common technique in order to characterize the mechanical properties of thin hard coatings on various types of substrate materials. In this test the normal load applied on the scratching stylus is continuously increased while the stylus is moved relative the surface. The critical load, corresponding to a cohesive and/or adhesive coating failure is registered either by a change in the force, the acoustic emission signal or preferable by combining the information from signals with post-test characterisation of the scratch using scanning electron microscopy.
The present study evaluates the possibilities to use scratch testing as a method to measure or at least quantitatively classify the adhesion characteristics of different types of oxide scales thermally grown on ferritic stainless interconnect steel. Both uncoated and interconnect steel pre-coated with thin metallic coatings are studied. The results show that all oxide layers investigated display a sufficient cohesive strength and adhesion to the underlying substrate and that the major scratching induced surface failure mechanisms are plastic deformation and cracking, i.e. no brittle like chipping or spalling could be observed.

20.

Bexell, Ulf

et al.

Dalarna University, School of Technology and Business Studies, Material Science.

Ferritic stainless steel has attracted a great deal of attention for its use as an interconnector in solid oxide fuel cells (SOFCs). The ferritic Sandvik Sanergy HT chromium steel is specially developed for interconnectors in SOFC with a unique chemical composition, which gives the alloy a good high temperature corrosion resistance as well as good surface conductivity in the formed chromium oxide scale. However, chromium evaporation from metallic interconnectors in SOFC fuel cells tends to poison the cathode of the fuel cell. Furthermore, the evaporation of chromium species from the oxide surface tends to increase the oxidation rate resulting in increased contact resistance. It is nowadays well known that thin coatings of e.g. cobalt can substantially reduce the chromium evaporation from the interconnectors.
In this study, a 22% Cr ferritic steel, Sandvik Sanergy HT has been coated with a thin metallic cobalt film. The coated material was plastically deformed and the effect of chromium evaporation was studied as a function of the degree of deformation. Coated samples are also compared to uncoated material. The results show that the metallic cobalt film cracks when the steel sheet is formed but the chromium evaporation is more or less unaffected.

21.

Bexell, Ulf

et al.

Dalarna University, School of Technology and Business Studies, Material Science.

The influence of annealing time on an amorphous As cap layer and the depth distribution of Mn atoms have been investigated. The results show that a 1600 Å thick As cap layer is completely desorbed after 3 h of annealing time. The depth distributions of Mn indicate that interstitial Mn atoms have diffused to the outer surface and being passivated. The thickness of the Mn passivation layer was around 90 Å.

Images obtained by scanning electron microscopy (SEM) helped to clarify the question as to how anatomy influences the deformation on compression and the spring-back of densified wood on water soaking. Transverse sections of Norway spruce (Picea abies), Scots pine (Pinus sylvestris), black alder (Alnus glutinosa), Swedish aspen (Populus tremula), European birch (Betula pubescens), European beech (Fagus sylvatica) and pedunculate oak (Quercus robur) were studied. Wood is reinforced with rays in the radial direction and with dense latewood in the tangential direction. When strained radially, rays buckle or tilt tangentially. Softwoods were mainly compressed radially, owing to low number of rays and since latewood is much denser than earlywood. The diffuse-porous hardwoods with low density variation between latewood and earlywood were mainly deformed tangentially, except birch, which has high density at the annual ring border and is mainly compressed radially. The ring-porous hardwoods were relatively equally deformed in the radial and tangential directions because of the high number of rays and high latewood density. Moisture-induced spring-back (shape recovery) was proportional to the degree of compression. Rays remained deformed, which also influenced the surrounding wood. Longitudinal wood cells almost resumed their original shape. Wood with low density and a low degree of compression showed the highest structural recovery. Shearing deformation was particularly pronounced and permanent in woods with high strength anisotropy. Thin-walled and sheared cells, such as earlywood in softwood, tended to crack on compression. Cracks usually stopped at the middle lamella and had a lesser influence on strength properties than for lumen-to-lumen cracks.

Adhesive wear, generally defined as ‘wear due to localised bonding between contacting solid surfaces leading to material transfer between the two surfaces or loss from either surface’ is a common phenomenon in many sliding contact tribosystems, e.g. sheet metal forming operations. In these operations, galling, i.e. seizure of the sheet surface caused by transfer of sheet material to the tool surface, is frequently a problem since it may results in scratching of the formed sheet surface and eventually cracking and fracture of the product due to high friction forces.
In order to reduce the coefficient of friction and the galling tendency in sheet metal forming operations thin organic coatings has been introduced on the market with the intention of improving the performance of hot-dip coated steel sheet. In summary, these coatings have the potential to increase the formability without additional lubrication and serve as temporary corrosion protection during transportation. In the present study, the friction and wear mechanisms of five different thin organic permanent coatings deposited on hot-dip coated (Zn and 55% Al–Zn) steel sheet is evaluated by modified scratch testing.
The results obtained show that this test method permits easy and reproducible evaluation of the tribological properties of thin organic coatings. Further, these coatings show a high potential when it comes to improve the formability of hot-dip coated steel. The results obtained are discussed in relation to the identified friction and wear mechanisms.

26.

Carlsson, Per

et al.

Dalarna University, School of Technology and Business Studies, Material Science.

Bexell, Ulf

Dalarna University, School of Technology and Business Studies, Material Science.

Olsson, Mikael

Dalarna University, School of Technology and Business Studies, Material Science.

The forming and handling of hot-dip coated steel sheets is frequently associated with problems such as galling, scratching and discoloration. Recently, a new generation of thin organic coatings has been introduced on the market in order to improve the performance of hot-dip coated steel sheets and reduce these kinds of problems. In summary, these coatings have the potential to increase the formability of the steel sheet without additional lubrication, the anti-finger print properties and the corrosion protection of the product. Besides, they should also provide a pre-treatment for painting, i.e. they can be classified as permanent coatings. In the present study, the tribological behaviour of three different thin organic permanent coatings deposited on hot-dip coated (pure zinc and 55% Al–Zn) steel sheets is evaluated by three different laboratory tests; modified scratch testing, pin-on-disc testing and bending under tension testing. The results obtained show that all tests yield consistent and valuable information concerning the friction and wear properties of the materials and can, therefore, be used in order to study the tribology in sheet metal forming and the performance of different types of permanent coatings. Of the permanent coatings investigated, a pure organic coating shows the lowest coefficient of friction (µ close to 0.1) and the highest wear resistance, thus offering excellent anti-galling properties. In contrast, a mixed organic/inorganic coating displays a relatively high coefficient of friction (µ close to 0.3) and a significantly lower wear resistance. Surface analyses of the tested surfaces show that the thickness and coverage of the thin organic coating play an important role in controlling friction and wear. Furthermore, a thin organic coating optimized for improved formability and handling should display: a high adhesion to the underlying substrate material, a low coefficient of friction, a high load carrying capacity and a high intrinsic wear resistance.

27.

Carlsson, Per

et al.

Dalarna University, School of Technology and Business Studies, Material Science.

Bexell, Ulf

Dalarna University, School of Technology and Business Studies, Material Science.

Olsson, Mikael

Dalarna University, School of Technology and Business Studies, Material Science.

Dry lubricants are today increasingly being used in various types of sheet metal forming operations. Among these, permanent coatings, based on organic resins are the only lubricants which have the potential to increase the formability without additional lubrication, serve as temporary corrosion protection during transportation and, finally, serve as a pre-treatment before subsequent painting.
In the present study, the influence of coating composition and thickness on the friction and wear behaviour of different thin organic permanent coatings deposited on 55%Al–Zn coated steel sheet have been evaluated by various types of laboratory tests. Surface profilometry, scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectrometry (ToF–SIMS) were used in order to characterise and model the tribological behaviour of the coatings.
The results obtained show that the tribological properties of thin organic permanent coatings are strongly influenced by the coating thickness. In order to reduce problems associated with high friction and galling, the coating must be deposited with a uniform thickness, i.e. uncoated regions must be avoided. Furthermore, the addition of various types of additives can be used in order to further improve the tribological performance of these types of coatings.

28.

Carlsson, Per

et al.

Dalarna University, School of Technology and Business Studies, Material Science.

Bexell, Ulf

Dalarna University, School of Technology and Business Studies, Material Science.

Olsson, Mikael

Dalarna University, School of Technology and Business Studies, Material Science.

Hot-dip zinc coated steel sheet is extensively used to improve the corrosion protection of steel constructions. When the sheet is formed cracks in the zinc coating develop in strained areas. The zinc coating gives a galvanic protection of the steel in damaged areas of the coating and at cut edges of the sheet. The degree of protection is, however, dependent on factors such as the geometry and the area of the defects, the coating thickness, the presence of corrosive ions in the electrolyte and the wet time. In this work we have studied the initial atmospheric corrosion of zinc coated steel in defects on bended and scribed material. The samples were exposed to a cyclic indoor corrosion test developed by Volvo (Volvo standard 1027). Scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and Auger electron spectroscopy (AES) were used to monitor the initial stages of corrosion and the growth of corrosion products. The corrosion products were identified as predominantly zinc hydroxycarbonate, zinc hydroxychloride and zinc hydroxide. The amount of corrosion products increases with the size of the damaged area, which suggests that the cathodic reduction of oxygen at the steel substrate controls the corrosion rate.

29.

Carlsson, Per

et al.

Dalarna University, School of Technology and Business Studies, Material Science.

Bexell, Ulf

Dalarna University, School of Technology and Business Studies, Material Science.

Olsson, Mikael

Dalarna University, School of Technology and Business Studies, Material Science.

The superior high temperature oxidation resistance of FeCrAl alloys relies on the formation of a dense and continuous protective aluminium oxide layer on the alloy surface when exposed to high temperatures. Consequently, the aluminium content, i.e. the aluminium concentration at the alloy–oxide layer interface, must exceed a critical level in order to form a protective alumina layer. In the present study the oxidation behaviour of six different FeCrAl alloys with Al concentrations in the range of 1.2–5.0wt% have been characterised after oxidation at 900 8C for 72 h with respect to oxide layer surface morphology, thickness and composition using scanning electron microscopy, energy dispersive X-ray spectroscopy and Auger electron spectroscopy.The results show that a minimum of 3.2wt% Al in the FeCrAl alloy is necessary for the formation of a continuous alumina layer. For Al concentrations in the range of 2.0–3.0wt% a three-layered oxide layer is formed, i.e. an oxide layer consisting of an inner alumina-based layer, an intermediate chromia-based layer and an outer iron oxide-based layer. In contrast, the 1.2wt% Al FeCrAl alloy is not able to form a protective oxide layer inhibiting extensive oxidation.

The high temperature oxidation characteristics of uncoated and SiO2 PVD-coated FeCrAl foils have been investigated when exposed to laboratory air at 1000 °C during 1, 2, 4, 8, 16, 32 and 60 min. The oxidized samples were characterized using SEM, EDS, AES and SIMS. The results show that the presence of a 100 nm thin SiO2 PVD coating significantly reduces the oxidation rate of the FeCrAl foil during early stages of oxidation. The decreased oxidation rate displayed by the SiO2 coated FeCrAl foil is the result of the SiO2 coating acting as an initial diffusion barrier promoting the formation of a predominantly inward growing Al2O3 layer during oxidation. Additionally, by using EDS analysis together with AES and SIMS depth profiling it was shown that the total concentration of Si in the grown oxide scale decreased during oxidation.

A FeCrAl alloy was preoxidized to form a protective alumina scale, and the effect of KCl deposits on the alumina scale was investigated while exposed during 1 and 24 h at 600 °C. However, impurity concentrations in the alumina scale change the ion conductivity and hence affect its protective properties. Therefore, Auger electron spectroscopy and time-of-flight secondary ion mass spectrometry were used to characterize the alumina scales and detect traces of K and Cl. The results showed that K and Cl existed as large sharp-edged crystals surrounded by a dendritic network after 1 h. However, after 24 h, those were dissolved, and K together with Cl was detected only in the outer layer of the duplex alumina scale, which was formed during preoxidation.

Anodic oxidation was used to grow porous layers on titanium discs. Six different oxidation procedures were used producing six different surfaces. The implants were inserted in rat bone (tibia) for 7 days. After implant retrieval, mineralization (hydroxyapatite formation) on the implant surfaces was investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Bone tissue around the implants was sectioned and stained. The amount of bone in close apposition to the implant was calculated. The porosity showed great variation between the surfaces. Hydroxyapatite was detected on all surfaces. A slight positive correlation between porosity and mineralization was found, although the most porous surface was not the best mineralized one. Bone had formed around all implants after 7 days. The bone-to-metal contact for the porous implants did not differ significantly from the non-porous control. Porosity is known to influence cellular events. The results indicate that porosity could have an initial, positive influence on bone integration of implants, by stimulating the mineralization process. The methods used were found to be suitable tools for investigation of initial healing around implants in bone.

The drying process of linseed oil, oxidized at 80 oC, has been investigated with rheology measurements, Fourier transformation infrared spectroscopy (FTIR), and time of flight secondary ion mass spectrometry (ToF-SIMS). The drying process can be divided into three main steps: initiation, propagation and termination. ToF-SIMS spectra show that the oxidation is initiated at the linolenic (three double bonds) and linoleic fatty acids (two double bonds). ToF-SIMS spectra reveal peaks that can be assigned to ketones, alcohols and hydroperoxides. In this article it is shown that FTIR in combination with ToF-SIMS are well suited tools for investigations of various fatty acid components and reaction products of linseed oil.

The initial stages of metal-organic chemical-vapor deposition of ZrO2 on a model FeCrAl alloy was investigated using synchrotron radiation photoelectron spectroscopy, X-ray absorption spectroscopy, scanning Auger microprobe, and time of flight secondary mass spectrometry. The coatings were grown in ultra-high vacuum at 400 °C and 800 °C using the single source precursor zirconium tetra-tert-butoxide. At 400 °C the coatings mainly consist of tetragonal ZrO2 and at 800 °C amixed ZrO2/Al2O3 layer is formed. The Almetal diffuses from the FeCrAl bulk to themetal/coating interface at 400 °C and to the surface of the coating at 800 °C. The result indicates that the reactionmechanism of the growth process is different at the two investigated temperatures.

MoS2 and WS2 are widely known intrinsic low-friction materials that have been extensively used and thoroughly investigated in literature. They are commonly produced in the form of sputtered coatings and show extremely low friction coefficients in non-humid environments, but rapidly degrade in humid conditions. Close nested fullerene-like nanoparticles of these materials have been proposed to have better oxidation resistance due to their closed form with the absence of dangling bonds. In the present study, an electrochemically deposited coating consisting of fullerene-like nanoparticles of WS2 embedded in a Ni-P matrix is tested under various loads and humidity conditions and compared with a sputtered WS2 coating with respect to their tribological behavior. The formation of a tribofilm on both surfaces is known to be crucial for the low-friction mechanism of WS2 and the different mechanisms behind this formation for the two types of coatings are investigated. It is shown that despite the completely different transformation processes, the resulting tribofilms are very similar. This is analyzed thoroughly using SEM, AES and TEM. The friction coefficient is known to be lower at higher normal loads for these materials and in the present study the mechanical and chemical responses of the tribofilm to higher normal loads during sliding are investigated. It was observed that the basal planes become aligned more parallel to the surface at higher loads, and that the tribofilm is less oxidized. It is suggested that these mechanisms are connected and are crucial keys to the wear life of these materials. (C) 2013 Elsevier B.V. All rights reserved.

Oxide scale cross-sections of CeO2 coated FeCr based solid oxide fuel cell interconnect materials were examined using secondary ion mass spectrometry (SIMS) depth profiling. A duplex spinel∶chromia scale was formed after 1 h at 850°C. Ti and ceria were observed between these layers. Additionally, minor concentrations of Mn, Si and Nb were observed at the oxide/metal interface. Furthermore, Al and Ti were concentrated primarily in the metal surface close to the oxide/metal interface. Secondary ion mass spectrometry sputter depth profiles using different ion sources; 69Ga+, Bi3+/Cs+ and C60+/C602+ were compared with TEM oxide scale cross-section and field emission gun–Auger electron spectroscopy depth profiling. Secondary ion mass spectrometry depth profiling with 69Ga+, Bi3+/Cs+ showed decreased secondary ion yields in the metallic matrix. This decrease could be avoided using oxygen flooding. The C60cluster ion depth profiles were less sensitive to type of matrix and gave the best correspondence to the TEM cross-section. However, the impact energy has to be high enough to avoid carbon deposition.

The influence of surface topography of PVD coatings on the initial material transfer tendency and friction characteristics in dry sliding contact conditions has been investigated. A modified scratch test was used to evaluate the material transfer tendency between ball bearing steel and two different PVD coatings, TiN and WC/C, under dry sliding contact conditions. Post test characterisation of the contact surfaces was performed using SEM/EDS and AES in order to map the initiation points and mechanisms for material transfer. The results show that the resulting topography of the PVD coated surfaces is strongly dependent on both the substrate material topography and the topography induced by the coating deposition process used. In sliding contact with a softer surface the coating topography results in a significant material pick-up tendency of the PVD coated surfaces. The material pick-up is mainly controlled by the abrasive action of hard coating asperities and as a result a polishing post treatment of the as-deposited PVD coatings significantly reduces the material pick-up tendency. For the WC/C coating, showing intrinsic low friction properties, the post treatment inhibits the material pick-up and results in a low and stable friction coefficient (mu similar to 0.1). For the TiN coating, that lacks intrinsic low friction properties, the post treatment reduces the material pick-up tendency but has no significant influence on the friction characteristics. This is mainly due to the presence of metallic Ti originating from the macroparticles on the TiN coating which results in a reactive surface that promotes a strong adhesion between the mating surfaces.

The influence of surface roughness on the tribological performance, i.e. friction, wear and material pick-up tendency, of two different commercial PVD coatings, TiN and WC/C, in sliding contact with ball bearing steel has been evaluated using two different types of sliding wear laboratory tests. Post-test characterisation using SEM/EDS, AES, ToF-SIMS and XPS was used to evaluate the prevailing friction and wear. The results show that the surface roughness of the coating is of importance in order to control the initial material pick-up tendency and thus the friction characteristics in a sliding contact. Once initiated, the material pick-up tendency will increase, generating a tribofilm at the sliding interface. For steel–TiN sliding couples a FeO-based tribofilm is generated on the two surfaces and FeO/FeO becomes the sliding interface (interfilm sliding) resulting in a high friction coefficient. For steel–WC/C sliding couples the WC/C displays a pronounced running-in behaviour which generates a WO3-based tribofilm on the steel surface while a carbon rich surface layer is formed on the WC/C surface, i.e. WO3/C becomes the sliding interface (interface sliding) resulting in a low friction coefficient.

A sliding test is used in order to evaluate the influence of tool surface roughness on the material transfer in intermittent and continuous sliding of PVD coated HSS against case hardening steel (20NiCrMo2). Two cutting tool coatings, TiN and AlCrN, and three different surface roughnesses are tested. For polished surfaces the same types of material transfer are obtained irrespective of sliding mode and coating type. If the surfaces are too rough, the tribofilms do not grow thick enough to separate the surfaces and the work material is abrasively worn in both sliding modes. With increased sliding distance, cracking of the TiN coating occurs while the AlCrN coating remains intact. (C) 2012 Elsevier Ltd. All rights reserved.

An intermittent sliding test was used in order to study the formation and build- up of tribofilms during intermittent sliding of PVD coated HSS against case hardening steel (20NiCrMo2). Two cutting tool coatings were tested, TiN and AlCrN, and the influence of sliding speed was evaluated. With moderate speed, two tribofilms were formed separately, one consisting of Mn, Si, Al and O on an intermediate layer of Fe and one consisting of Fe, Mn, Cr and O on an intermediate layer of Cr and Mn. At low sliding speeds an uneven transfer of steel occured while high sliding speeds resulted in thermal softening of the substrate leading to coating failure. AlCrN provided better substrate protection at high speeds than TiN did.

This paper reports on hydrogen pressures measured during similar to 19,000 h immersion of copper in oxygen-free liquid distilled water. Copper corrosion products have been examined ex-situ by SEM and characterized by XPS and SIMS. XPS strongly indicates a corrosion product containing both oxygen and hydrogen. SIMS shows that oxygen is mainly present in the outer 0.3 mu m surface region and that hydrogen penetrates to depths well below the corrosion product. Thermal desorption spectroscopy shows that the reaction product formed near room-temperature is less stable than that formed in air at 350 degrees C.

This paper reports on hydrogen pressures measured during the longterm immersion (~19 000 hours) of copper in oxygen-free distilled water. Hydrogen gas evolution is from copper corrosion and similar pressures (in the mbar range) are measured for copper contained in either a 316 stainless steel or titanium system. Copper corrosion products have been examined ex-situ by SEM and characterized by Xray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). XPS strongly indicates a corrosion product containing both hydroxide and oxide. SIMS shows that oxygen is mainly present in the outer 0.3 um surface region and that hydrogen penetrates to depths in the substrate well below the corrosion product.

48. Hörnström, Sven Erik

et al.

Bexell, Ulf

Dalarna University, School of Technology and Business Studies, Material Science.